The human ear may be described in three parts as illustrated in FIG. 1A. The first part is the external ear 101 which includes pinna 102 for receiving and concentrating sound pressure waves to the ear canal 103. The second part is the middle ear 104 which receives the sound pressure waves from ear canal 103. The waves pass through eardrum 105 and are transformed into mechanical vibrations that are transmitted to the ossicular chain. The vibrations pass through malleus 106, then incus 107 and finally stapes 108. The third part of the human ear is known as the inner ear or cochlea 109. A cross-section of the cochlea 109 is shown in FIG. 1B. The vibrations of the ossicular chain are transmitted from the stapes 108 to the perilymph liquid of the scala vestibuli 136 through the oval-shaped windows of the cochlea 109. Once in the cochlea 109, the vibrations in the liquid initiate the organ of Corti 110 to generate stimulus in the acoustic nerve 138.
One type of existing hearing aid device is known as a cochlear implant. As shown in FIG. 1C, a cochlear implant 142 comprises two main parts being an external device 111 (also referred to as a speech processor) and an implantable device 113. Speech processor 111 is composed of a transmitting coil 112, a microphone, electronics, and a battery (not shown). The implantable device 113 includes a housing 114, an electronic and separate receiving coil 115, and a generally flexible lead 116 to connect the housing 114 to an electrode array 117. The flexible lead 116 passes through the middle ear 118 to reach the cochlea 119. As shown in FIG. 1D, in most instances an electrode array 121 is composed of an electrode carrier 122 made of a flexible material such as silicone in order to facilitate insertion of the array 121 into the scala tympani 120 of the cochlea 119. The scala tympani 120 has a helical shape 123 and the flexible material of carrier 122 allows for a folding effect. The electrode carrier 122 contains connection wires (not shown) that are connected to stimulation electrodes 124 made of platinum, platinum-iridium or other material having a biocompatible surface treatment.
FIG. 1E illustrates an example of an electrode array 144 composed of cylindrical electrodes 125 made of platinum-iridium. Each electrode 125 is separated from each other by a small silicone ring 126. The diameter of each silicone ring 126 regularly decreases towards proximal end 127 in order to form a progressively finer or tapered surface. Additional rings 128 are disposed on the basal diameter of the electrode array 144 to assist insertion of the array 144 into the cochlea. Generally, the electrode array of existing cochlear implants has an average of one electrode per each one millimeter along the array.
A problem with existing cochlear implant systems is the realization of spatially high-resolution electrical stimulation to restore sound perception close to normal hearing. The stimulating current usually diffuses and stimulates a large number of spiral ganglia with low specificity if the electrode array is located far from the target cells. It is thus important to locate the electrode array close to the spiral ganglia in the cochlea (the target of the electrical stimulation). As described above and in FIG. 1D, the electrode array of a cochlear implant is generally inserted into the scala tympani because it has the largest cross-sectional area and is an easy surgical operation site. Stimulating currents flowing from the electrode site stimulate the spiral ganglion cells located in the direction of the cochlea modiolus. Therefore, it is desirable to locate the electrode array of a cochlear implant system close to the modiolus to reduce the distance between the electrode and the spiral ganglia and increase the spatial specificity of the electrical stimulation.
Several strategies have been developed and employed in cochlear implant systems to locate the electrode close to the modiolus. For example, modiolus-hugging electrodes are developed, these electrode use a carrier which is held straight by an internal stylet before and during the insertion process in the scala tympani. After the partial insertion of the electrode and stylet, the electrode is pushed to its full insertion depth while holding the stylet at the same position. The electrode returns to its original spiral shape while being pushed toward the modiolus. Accordingly, the insertion depth of the stylet is critical in modiolus-hugging electrode array surgery. If the stylet is inserted far deeper into the cochleostomy, the electrode tip will come into contact with the cochlear outer wall, which may damage the spiral ligament or penetrate into the scala vestibuli. On the other hand, if the insertion depth of the stylet is too short, the apical curve of the electrode will curl before the first turn of the scala tympani. Thus, previous efforts to facilitate current flow through the modiolus included fabrication and use of precurved electrodes designed to “hug” the modiolus and silastic positioners designed to place the electrodes closer to the modiolus.
Another aspect about electrical stimulation of the auditory system includes completion of a circuit loop whereby one electrode along the intracochlear electrode array serves as the active electrode and a second serves as a return electrode. Monopolar stimulation mode is the most commonly used strategy where the second extracochlear return electrode is located on the case of the receiver-stimulator unit. Bipolar stimulation mode, on the other hand, uses a neighboring electrode within the implanted intracochlear electrode array as the return. Various variations such as bipolar+1 or bipolar+2 are also possible. Other modes may include tripolar stimulation mode where current is delivered to one intracochlear electrode and its two neighboring electrodes serve as return electrodes. With different stimulation modes, one can achieve some advantages or face challenges such as in the area of power consumption and specificity of stimulation.
None of the currently available solution overcomes the above-mentioned shortcomings. Therefore, there is a need to provide a solution that allows for an extremely high number of contact electrodes in an array and/or to provide an alternative to current stimulation modes.